The classical explanation is that each species has evolved morphological or physiological traits that allow it to exploit some resources more efficiently than all other species. Such partitioning of resources essentially provides each species with exclusive access to resources necessary for its survival.
Although coexistence is often attributed to interspecific differences in morphology, direct evidence is relatively rare. Dabbling ducks, which include the ubiquitous mallard, are a good example. Dabbling ducks are primarily filter-feeders.
They use lamellae, which are comb-like projections on the bill, to sieve food particles from pond water. Many ecologists, including Darwin, suggested that ducks coexist because interspecific differences in the spacing of bill lamellae allow each species to consume food particles of different sizes. Research published in the March issue of the American Naturalist by Brent Gurd of Simon Fraser University has demonstrated that interspecific differences in lamellar length, not spacing, allow ducks to partition food by size.
"Lamellar spacing alone does not lead to resource partitioning," said Gurd, "ducks with small spacing are more efficient than species with wide spacing because they retain a wider range of particle sizes than species with wide spacing. In order for variation in bill morphology to lead to resource partitioning, each trait must impose both costs and benefits on foraging ducks. It is the trade-off between these costs and benefits that allows resources to be partitioned."
"Foraging ducks are faced with the cost of separating food particles from detritus particles like sand and silt" said Gurd. "To do this, they alter the position of the upper and lower bill while they are feeding. This alters the size of the gap between the lamellae on the upper and lower bill, which allows them to determine the size of the particles they filter and ingest. By avoiding particle sizes that contain too much detritus, ducks increase their foraging efficiency."
However Gurd found that avoiding detritus reduces the rate at which ducks can filter water. "It is this trade-off between avoiding detritus and foraging rate that allows ducks to partition resources" said Gurd. "Ducks with long lamellae are more efficient at selecting smaller food particles while ducks with short lamellae, like mallards, are more efficient at selecting larger particles."
To test his idea, Gurd created virtual bills using computer software typically used by engineers to design complex machinery. "The software allowed me to create exact, three-dimensional replicas of duck bills complete with articulating joints" said Gurd. "The replicas allowed me to determine the particle sizes each duck could ingest and the rate at which they could ingest them."
D. Brent Gurd, "Predicting resource partitioning and community organization of filter-feeding dabbling ducks from functional morphology" American Naturalist 169:334-343 (2007)
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